1. Field of the Invention
The present invention relates to a liquid lens with variable refractive power and an apparatus incorporating the liquid lens.
2. Description of the Related Art
Liquid lenses have been studied and developed as a form of lens with variable refractive power.
Among several types of liquid lenses, a liquid lens which utilizes the principle of electrowetting has been intensively studied. The electrowetting effect has been defined as the change in solid-electrolyte contact angle due to an applied potential difference between the solid (electrode) and the electrolyte. Thus, electrowetting involves modifying the surface tension of liquids on a solid surface using a voltage. By applying a voltage, the wetting properties of a hydrophobic surface can be modified and the surface becomes increasingly hydrophilic (wettable). Applying this principle to a liquid lens, an angle between an interface between an electrolyte liquid and a non-electrolyte liquid and a solid member which the two liquids are in contact with (hereinafter, this angle is referred to as a “contact angle”) is varied when a voltage is applied between the electrolyte liquid and an electrode.
Such electrowetting lenses are considered to be beneficial for certain imaging applications due to their rapid operation, sufficient surface accuracy of the interface, and the potential of low manufacturing cost resulting from the reduction in the size of the lens and the reduction in the number of parts.
The electrowetting lens incorporates an electrically conductive electrolyte liquid and a non-conductive non-electrolyte liquid with different refractive indices. The conductive electrolyte liquid and the non-conductive non-electrolyte liquid are immiscible liquids, that is, do not mix with each other but instead form a contact interface therebetween. Since these two liquids are sealed, application of a voltage between the electrolyte liquid and an electrode layer, which is disposed via an insulating layer, changes a contact angle of an end of the interface while the volume of the liquids remains unchanged.
The change in the contact angle causes a change in the sphere radius of curvature of the interface in accordance with the contact angle and a difference between the refractive indices of the two liquids induces a change in the optical refractive power. These two liquids typically are the same in density because a difference in density causes a distorted shape of a spherical surface of the interface due to the influence of gravity which leads to insufficient optical performance.
PCT Japanese Translation Patent Publication No. 2001-519539 (hereinafter “Patent Document 1”) discloses an invention to reduce inconvenience of a deviation of the center of a droplet of an insulating liquid from its original central axis due to a deformation which the droplet undergoes when a focal length of a lens is varied by the deformation of the liquid in an electrically conductive liquid as a result of application of a voltage to the electrically conductive liquid.
Patent Document 1 discloses causing a change, on a region basis, in “wettability” of an inner wall of a dielectric chamber which the electrically conductive liquid and droplets of the insulating liquid disposed in the electrically conductive liquid are in contact with. In particular, Patent Document 1 discloses a liquid lens of which an inner wall surface of a chamber is processed to reduce “wettability” to the electrically conductive liquid in a radial direction toward the central axis O.
Japanese Patent Laid-Open No. 2007-293349 (hereinafter “Patent Document 2”) discloses a liquid lens of which the shape of a contact surface which a liquid is in contact with is devised such that a change in curvature caused by application of a voltage is made in a more or a less sensitive manner.
In particular, Patent Document 2 implies that a change in the shape of a contact surface in which a liquid constituted by a base surface of an electrical conductive material coated with an insulating layer (i.e., an electrically conductive liquid and a non-electrically conductive liquid) may cause a change in curvature of a lens: a convex contact surface can make the change in a less sensitive manner and a concave contact surface can make the change in a more sensitive manner.
Patent Document 1 attempts to align the center of the droplet constituted by the insulating liquid with the central axis O of the droplet by causing “wettability” of the inner wall surface of the chamber with respect to the electrically conductive liquid to be lowered in a radial direction toward the central axis O. However, no attempt is made to achieve predetermined refractive power when a low voltage is applied.
Patent Document 2 discloses increasing sensitivity to a change in curvature of lens in response to application of a voltage by devising the shape of the contact surface which a liquid is in contact with (specifically, not providing a linearly tapered sectional shape of a common ring electrode but providing a concave-curved surface).
However, in an attempt to obtain a liquid lens with a concave contact surface, regarding a section which includes a cylindrical optical axis and is parallel to the optical axis, it is necessary to provide a concave ring electrode and an insulating layer which constitute the contact surface: such a concave surface is difficult to process and sufficient accuracy is required to process the same. Such a liquid lens is expensive to manufacture and, therefore, is difficult to be used as a liquid lens suited for mass production.